The present invention pertains to the art of reducing off-focal radiation and collimating in connection with x-ray generation. It finds particular application in conjunction with annular x-ray tubes for CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with the generation of radiation for other applications.
Typically, a patient is positioned in a supine position on a horizontal couch through a central bore of a CT scanner. An x-ray tube is mounted on a rotatable gantry portion and rotated around the patient at a high rate of speed. For faster scans, the x-ray tube is rotated more rapidly. However, rotating the x-ray more rapidly decreases the net radiation per image. As CT scanners have become faster, larger x-ray tubes have been developed which generate more radiation per unit time to maintain the desired radiation dose at higher speeds. Larger tubes, of course, cause high inertial forces.
High performance x-ray tubes for CT scanners and the like commonly include a stationary cathode and a rotating anode disk, both enclosed within an evacuated housing. As more intense x-ray beams are generated, there is more heating of the anode disk. In order to provide sufficient time for the anode disk to cool by radiating heat through the vacuum to surrounding fluids, x-ray tubes with progressively larger anode disks have been built.
The larger anode disk requires a larger x-ray tube which does not readily fit in the small confined space of an existing CT scanner gantry. Particularly in a fourth generation scanner, incorporating a larger x-ray tube and heavier duty support structure requires moving the radiation detectors to a larger diameter. A longer radiation path between the x-ray tube and the detectors would require that the detectors be physically larger to subtend the required solid angle. Larger detectors would be more expensive. Not only is a larger x-ray tube required, larger heat exchange structures are required to remove the larger amount of heat which is generated.
Rather than rotating a single x-ray tube around the subject, others have proposed using a switchable array of x-ray tubes, e.g. five or six x-ray tubes in a ring around the subject. See, for example, U.S. Pat. No. 4,274,005 to Yamamura. However, unless the tubes rotate, only limited data is generated and only limited image resolution is achieved. If multiple x-ray tubes are rotated, similar mechanical problems are encountered trying to move all the tubes quickly and remove all of the heat.
Still others have proposed constructing an essentially bell-shaped, evacuated x-ray tube envelope with a mouth that is sufficiently large that the patient can be received a limited distance in the well of the tube. See, for example, U.S. Pat. No. 4,122,346 issued Oct. 24, 1978 to Enge or U.S. Pat. No. 4,135,095 issued Jan. 16, 1979 to Watanabe. An x-ray beam source is disposed at the apex of the bell to generate an electron beam which impinges on an anode ring at the mouth to the bell. Electronics are provided for scanning the x-ray beam around the evacuated bell-shaped envelope. One problem with this design is that it is only capable of scanning about 270.degree..
Still others have proposed open bore x-ray tubes. See, for example, U.S. Pat. No. 5,125,012 issued Jun. 23, 1992 to Schittenhelm and U.S. Pat. No. 5,179,583 issued Jan. 12, 1993 to Oikawa. These large diameter tubes are constructed analogous to conventional x-ray tubes with a glass housing and a sealed vacuum chamber. Such tubes are expensive to fabricate and are expensive to repair or rebuild in case of tube failure.
Copending U.S. application Ser. No. 08/224,958 discloses a ring anode disposed in the housing. An annular rotor is rotatably received in the toroidal housing. At least one cathode is mounted on the rotor for generating an electron beam which strikes the anode target. The rotor and the cathode are rotated such that the electron beam is rotated around the ring anode. X-rays are emitted from a ring anode that is struck by energetic electrons from one of selected cathodes on the rotor. The more precisely the x-rays are collimated into a fan or other preselected shaped beam, the sharper and more artifact free the resultant CT images are.
One consideration is the amount of off-focal radiation produced during generation of the x-ray beam. Off-focal radiation is produced primarily due to energetic backscattered electrons whose energy is comparable to x-rays in locations off of the focal spot. The backscattered electrons tend to cause x-rays to be generated from broad areas of the x-ray anode and any surrounding material that may be at a positive potential relative to the cathode. Off-focal radiation has a negative effect on the image quality of x-ray images, particularly the reconstructed CT images. The off-focal radiation is a broad source of radiation that tends to blur in CT images and to cause more pronounced artifacts in the region of the interface of high and low contrast objects.
Moreover, a fixed collimator attached to the rotating frame within the vacuum enclosure is one method that can create the required fan beam of x-rays. This approach is most effective for fixed single or multiple slice applications in which each cathode assembly has an individual collimator. It is difficult and inconvenient to adjust collimators within a high vacuum. Mechanical adjustment mechanisms increase a risk of vacuum contamination.
The present invention contemplates a new and improved toroidal x-ray tube and CT scanner which resolves the above referenced difficulties and others.